Photovoltaic devices convert the energy of sunlight directly into electricity by the photovoltaic effect. Photovoltaic devices include semiconductor material deposited over a substrate, including, for example, a semiconductor emitter or window layer adjacent to a semiconductor absorber layer. The semiconductor window/emitter layer allows the penetration of solar radiation to the absorber layer, which converts solar energy to electricity. A conductor may be deposited adjacent to the semiconductor absorber layer to serve as a back contact for the device. A transparent front contact may be deposited on the side of the window/emitter layer. Since layer thickness has a substantial impact on device performance, it is desirable to accurately measure layer thickness and control the deposition thickness of one or more layers in the photovoltaic device in real time.